BHP says having to remotely derail a train led to a $US600 million drop in iron ore production.

The company had to force a fully-laden train pulling 268 wagons and 30,000 tonnes of iron ore off the tracks 120km south of Port Hedland, sending the order from its Integrated Remote Operations Centre in Perth, about 1,500km away.

The incident left the train in a mangled heap and did not do much for the balance sheet either.

It hit BHP's iron ore production by four mega tonnes, or about 6 per cent – the single biggest factor in lower-than-expected volumes reported for the December quarter.

“Productivity for the December 2018 half year has been impacted by unplanned production outages at Olympic Dam, Spence and (Western Australia Iron Ore), with a total negative impact of approximately $US600 million,” BHP's half-yearly operational review states.

“During the rail outage, mine stockpile levels were built and are expected to be partially drawn down in the March 2019 quarter.”